Various light projection systems are known. One particular system that is used for producing special lighting effects uses a powerful light source to project light through a metal slide (called a "gobo") which is then focused to produce an image. The light beam can be moved by a motorized mirror. In addition, the gobo itself can be rotated. A number of gobos can be loaded into a projector and selected at will. The gobos have different patterns so that a variety of patterns of different shapes can be projected onto a wall or other screen as well as to provide varied beam profile effects. However, the images that can be projected are limited to the particular gobos that are being used. There is no possibility of generating sophisticated images. There is obviously also no possibility of projecting video images when slides are used for carrying the image.
Attempts have been made in the past to project light through a liquid crystal display so as to produce a projection image that can be varied by appropriate control of the liquid crystal cells/pixels that make up the liquid crystal display. However, the problem with liquid crystal displays is that polarizing plates are placed either side of the liquid crystal cells, the front polarizing plate being conventionally known as the polarizer and the rear polarizing plate being conventionally known as the analyzer. Unpolarized light is caused to be incident on the polarizer. Only light having the appropriate planar polarization passes through the polarizer and light of the orthogonal planar polarization is absorbed by the polarizer so that it is not transmitted. However, this absorption of half of the incident light causes the polarizer to heat up significantly when a bright light source is used. The heat build up in the polarizer can cause heating of the liquid crystal in the liquid crystal cells by conduction of heat from the polarizer, causing breakdown of the device. Similarly, some of the light passing through the analyzer on the opposite side of the liquid crystal cells is absorbed by the analyzer, again causing heating of the liquid crystal and possible breakdown. Light intensity is lost because of the absorption of light by the polarizer. Increasing the intensity of the light source serves only to increase the undesirable heating in the system. Thus, it has not to date been possible to project images through a liquid crystal display that are bright enough for the resultant image to be projected for special lighting effects applications.
Notwithstanding the problems mentioned above, many systems have been described in the prior art for projection of light through a liquid crystal device. Many of such systems are for projection of video images (such as for a film or "movie"). However, in a video image projector, it is fair to say that on average each individual cell or pixel in the liquid crystal device is on for approximately half of the time and off for the other half of the time the device is used and thus heat build up in the analyzer is not a particular problem. In contrast, in a lighting effects system, a particular cell or pixel may be on or off for many minutes or even hours at a time. Accordingly, localized heat build up is potentially a major problem in a lighting effects system using liquid crystal displays. Furthermore, a lighting effects system often has to illuminate across large distances (perhaps a hundred meters or more when used in a stadium, for example), requiring the use of powerful light sources. In contrast, video image projectors are being proposed for use often in a domestic environment where the distance from the projector to the screen may only be a few meters, meaning that much lower power light sources can be used in a video image projector.
U.S. Pat. No. 5,172,254 discloses a light projector. Light from a source is split into two beams of different polarizations, which are then passed through their own respective liquid crystal devices. Light emerging from the respective liquid crystal devices is then recombined to produce a final image for projection onto a screen. A color system is also disclosed in which multiple liquid crystal devices are used. However, the use of plural liquid crystal devices makes the system difficult to set up as the liquid crystal devices for each beam must be very accurately aligned. This is an important issue in a special lighting effects projector that is often subject to rough handling in transit and in use.
U.S. Pat. No. 5,282,121 discusses the problem of heat build-up because of absorption by the polarizer in a liquid crystal display used in a lighting effects system. The solution to that problem as disclosed in the '121 patent is to use a scattering liquid crystal cell in the lighting effects system. However, the use of a scattering liquid crystal cell means that sophisticated cooling arrangements are required to keep down the temperature of the system components in the invention as disclosed in the '121 patent.
U.S. Pat. No. 5,283,600 discloses an LCD projector that uses a polarizing beam splitter to provide beams of light having orthogonal polarizations that are passed through a liquid crystal device having a polarizer and an analyzer.